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Python Math: How to Calculate Area of a Circle from Command Line

Calculating the area of a circle is a fundamental programming exercise that bridges basic math with input handling. In this guide, you will create a Python script that accepts a radius via command-line arguments, calculates the area using the math module, and formats the output to a specific precision.

Understanding the Logic​

To complete this task, we need to address three specific requirements:

  1. The Formula: The area of a circle is calculated as A = π r^2.
  2. The Input: Instead of hardcoding the radius or asking for input() during execution, we will pass the radius as a command-line argument (e.g., python script.py 10).
  3. The Output: The result must be formatted to exactly 10 decimal places.
tip

Python's math module provides a high-precision value for π via math.pi. Always use this constant instead of hardcoding 3.14.

Step 1: Importing Modules and Handling Input​

We need two standard libraries:

  • sys: To access arguments passed to the script via the command line.
  • math: To access the value of Pi (π).

accessing sys.argv​

The list sys.argv contains the command-line arguments.

  • sys.argv[0] is always the name of the script itself (CircleArea.py).
  • sys.argv[1] is the first argument passed by the user (the radius).
import sys

# ✅ Correct: Access the second element of the list (index 1)
# Note: Command line arguments are strings, so we must cast to float.
try:
    radius_arg = sys.argv[1]
    radius = float(radius_arg)
except IndexError:
    print("Error: Please provide a radius argument.")
    sys.exit(1)

Step 2: Implementing the Calculation​

Now we combine the input with the math module and format the result.

Complete Script: CircleArea.py​

Create a file named CircleArea.py and add the following code:

import sys
import math

# 1. Get command line argument (Radius)
# We assume the user provides a valid number as the first argument
radius = float(sys.argv[1])

# 2. Calculate the area of the circle
# Formula: Area = pi * r * r
area = math.pi * radius * radius

# 3. Format the result to 10 decimal places
# ".10f" means fixed-point notation with 10 digits after the dot
area_str = format(area, ".10f")

# 4. Print the result
print(area_str)
note

You can also calculate the square of the radius using the exponent operator: math.pi * (radius ** 2). Both methods are valid.

Step 3: Running the Script​

To test the script, you run it from your terminal, passing the radius value immediately after the filename.

Test Case 1: Radius 5​

python CircleArea.py 5

Output:

78.5398163397

Test Case 2: Radius 8​

python CircleArea.py 8

Output:

201.0619298297

Test Case 3: Radius 10​

python CircleArea.py 10

Output:

314.1592653590

Conclusion​

By using sys.argv, you can make your Python scripts dynamic and interactable from the shell.

  1. Import sys to read command-line inputs.
  2. Import math for accurate mathematical constants like pi.
  3. Use format(value, ".10f") to control the precision of your floating-point output.

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